Microwave filter insertable within outer wall of coaxial line



Jan. 26, 1965 R. b. HALL 3,167,729

MICROWAVE FILTER INSERTABLE WITHIN OUTER WALL OF COAXIAL LINE Filed Oct. 29, 1962 3 Sheets-Sheet 1 V L 1 2Ib 20 I E E INVENTOR.

ROBERT D. HALL ATTORNEY Jan. 26, 1965 R. D. HALL 3,167,729

MICROWAVE FILTER INSERTABLE WITHIN OUTER WALL OF COAXIAL. LINE Filed Oct. 29, 1962 3 Sheets-Sheet 2 W 1 z INVENTOR.

ROBERT D. HALL 8; gm W ATTORNEY Jan. 26, 1965 R. D. HALL 3,167,729

MICROWAVE FILTER INSERTABLE WITHIN OUTER WALL OF COAXIAL LINE Filed 001:. 29, 1962 3 Sheets-Sheet 3 FIE--15 INVENTOR.

ROBERT D. HALL ATTORNEY 3 I67 729 .MICROWAVE FILTER INSERTABLE WET OUTER WALL OF COAXIAL LINE Rohert D. Hall, Los Altos, Calif assignor to S ylvania Electric Products Inc, a corporation of Delaware Filed Oct. 29, 1962, Ee'r. No. 233,799

7 Claims; (Cl. 333---73) g This invention relates to electrical circuit components, and more particularly to a coaxial microwave filter.

Radio frequency filters comprising lumped reactance elements (capacitors and inductors) have an upper frequency limit which lies between 100 and 300 megacycles. Above this frequency range filter elements comprise lengths of open circuited and short circuited transmission lines. Thus, a system capable of transmitting frequencies up to 11 gygacycles might require two different types of circuit elements to form radio frequency filters. Also, filters using distributed elements must be of the order of a quarter wavelength at the frequencies at which they are employed and are therefore correspondingly large and. heavy. Moreover, because the elements used in suchfilters are transmission lines whose impedance characteristics are periodic functions of frequency, many filters also have periodic electrical characteristics which make them unsuitable for many applications.

An object of this invention is the provision of compact reactive circuit elements which are rapidly stacked to form a microwave filter without the use of special tools.

Another object is the provision of a microwave filter whose impedance characteristic is non-periodic.

"A further object is the provision of a filter which may be reproduced readily by'photographic and printed circuit techniques. I I a A specific object is the provision of a coaxial microwave filter which is extremely compact and which is wholly self-contained in a coaxial transmission line.

These objects are attained by the construction of four basic geometrical elements to provide series capacitance, shunt capacitance, series inductance and shunt induct: ance. The elements are made by forming ultra thin conducting layers of various geometrical shapes on insulator discs. The conductor geometry determines the nature (inductive or capacitive) of the reactance, its magnitude, and the type of connection (series or shunt). More than one type of reactance may be formed on a single disc. The discs are detachably assembled by axially stacking them in physical series arrangement within a coaxial line, the electrical series connected elements being inserted between a gap in the center conductor.

The outside diameter'of the coaxial'line is substantially unchanged.

The above and other objects'of this invention will be [come apparent from the following description of a preferred embodiment, reference being-had to the accompanying drawings in which:

, FIGURE 5 is a section taken on line .55 of FIG- I URE 4;

hilflfih Patented Jan. 26, 1965 ice FIGURE 6 is a longitudinal section of a coaxial filter V comprising'the element-s shown in FIGURES 2 and 4 arranged to form the pi-type low-pass filter of FIG- URE '1; f

FIGURE 7 is a schematic circui-t'diagram of a contional pi-type'high-pass filter;

FIGURE 8 is a .plan view of a shunt inductance disc element embodying the invention;

FIGURE 9 is a section takenon line 9-9 of FIG- FIGURE '11 is a section taken on line 11-11 of FIG- URE 1o; 7 Y

FIGURE 12 is a longitudinal section of an assembled coaxial filtericomprising elements shown in FIGURES 8 and 10 arranged to form the p'i-type high pass filter of FIGURE 7; i

FIGURE 13 is the coaxial filter-assembly of FIGURE 12 wherein the three disc reactive elements are combined on a single disc; 7

FIGURE 13a is a side view of a portion of the dis reactive element of FIGURE 13;

FIGURE 14 is a schematic circuit diagram of a conventional T-type high-pass filter; and

FIGURE 15 is a longitudinal section of a coaxial filter comprising a combination of disc reactive elements forming the T-type high-pass filter of FIGURE 14.

Referring now to the drawings, the filter shown in FIG URE 1 comprises a pair of conductors 1 and Z'Wllh a capacitor 4- connected in shunt between them, an inductor 5 connected in series with line 1, and another capacitor 6 connected in shunt between the conductors. This is a well-known configuration of a single section pi-type low-pass filter and is selected to illustrate one type. of filter which may be constructed in accordance with my invention. The broken line frames 4?, 5' and 6' indicate the corresponding filter elements in the filter constructed according to the invention and described below. I

In accordance with my invention, capacitor 4 takes the form of an insulator disc 8, see FIGURES 2 and 3, having a center opening 9. Conducting discs 11 and 12 are formed on opposite sides of disc 8. .T he outer diameter D of the larger disc 11 is thesame as the outer diameter of insulator'disc S and is designed to make contact with the outer conductor of the coaxial line in which the element is mounted.- The inner diameter d of conducting disc 11 is larger than the hole diameter d of the insulator disc 3. The other conducting disc 12 has an outer diameter D which lies between the inner and outer diameters of the larger disc so that both conducting discs overlap each other. The inner diameter of disc 12 is the same as the holediameter d of the insulator in order that electrical contact is made by disc .12 with the coaxial line inner conductor which also has 7 as shownin FIGURES'4 and 5. An insulator disc 14, similar. to disc 8, is fabricated with an opening near but spaced from the outer edge of the disc. Two conducting strips 16 and 17 are formed on opposite sidesof the disc .14 and-make electrical contact through thehole '15. The ends 16a andl7a' of thesestrips are located at approximately the center ofthe discin order to make contactwith the center conductor of the coaxial line in which the element is placed. The entire length and cross sectional area of the conductor comprising strips 16 and 17 determine'the inductance of the element and this length may be varied to meet this requirement by-curving the strips as shown. These conducting strips, like the discs 11 and 12*of the capacitor element 4', preferably are formed by etching and plating techniques and .in one embodiment had a thickness in the order of 0.002 inch and a width'of approximately 0.002 inch. The diameter of disc 14 is preferably the same as the outside diameter ofinsulator disc. 8. v

The filter assembly shown in FIGURE 6 comprises a coaxial line having a two-part outer conductor Zia and 21b and a two-part inner conductor 22aand 22b. The two parts of thecoaxial line are joined ,by means of connector assembly 32. Outer conducor Ziahas a terminal.

end portion formed with a recess 24 into which the-reactive disc elements are fitted. An externally threaded ring 27 is permanently secured to theend of outer conductor 21a for. engagement by nut 28 which mechanically connects adjacent outer conductor parts 21a and 21b. An annular boss30 on outer conductor 21b ofthe adjacent section of coaxial line provides a shoulder-against which nut 28 abuts when theparts are connected. Metals lic conducting ring 35 provides the coaxial line with an outer conductor having a continuous inner diameter and makes electrical contact between outer conductor parts 21a and 21b. i g

The filter assembly proper comprises spacers 33 and 34 which separate inductor element 5' from capacitor elements t and 6. Center conductor 22:: extends through I 4 ring '27. readily by. reversing this procedure.

In an actual embodiment of this invention, spacer discs 33 and 34 and insulator disc .14. of inductor element 5 were each made approximately 0.060 inch thick to reduce stray capacitance between conductive discs 12 of capacitor elements 4 and 6 and between conductive strips 16 and 17 of inductor element 5'2. The filter assembly is less than 0.250 inchthick.

Another type of filter which, byway of example, may be constructedin accordance .with my invention is a .pi-section high-pass rfilter as shown in FIGURE 7. This 'filter comprises lines 38 and 39, a series capacitor '41,

and shunt-inductor elements .40 and 42..v The broken line frames separating these reactive elements identify corresponding parts of the filter assembly shown in FIG- A shunt inductor element in accordance with my inconductionbands 47 and'48 are .formed on one .side

" of the disc- The outer band 47 preferably has the same the respective central openings in capacitor element 4 conductors 22a and 22b. The larger conducting disc 11 I of capacitor element 4' engages shoulder 25 and the inner surface of recess 24 to make electrical contact with outer conductor 21a. The larger conducting disc 11 of capacitor element 6' contacts metallic. ring whichis in elec trical engagement with the outer conductors 21a and 21b.

The several discs which comprise the filter are held firmly.

in position within. the coaxial line when connector assembly 32 is tightened. V I

The filter assembly describedabove may be readily constructed by simply inserting the reactive elements within the exposed recess 24 of outer conductor 2112 prior to the connection of the adjacent section of the coaxial line. The assembly-is completed by connecting the adjacent sections of coaxial line and inserting the adjacent center conductor 22b into the filter by tightening .nut 28 on outside diameter as insulator disc :45 which preferably is slightly larger than the; inside diameter of the outer conductor of 1 the. coaxial line into which the parts. are 'fitted'asdescribed below. Inner band 48 has an outsidediametersubstantiallylessthan that: of the outer band and. is formed on the midportion-of the same side of disc 45$ The inductance of this element is "provided by a conductive strip 49, shown in the drawing as a radial strip, which extends' between and makeselectrical contact with outer and innerv bands 47 and 48, respectively. The physical dimensions of conductive strip 49 determine the value of inductance of the element-and therefore this strip. may be curved or otherwise shaped to have a length and cross sectional area to give the desired inductance.

The series capacitor disc element 41' is'shown in'FIG- URES 10 and 11. This element comprises insulator. disc '51"havin'g conductorplates 521and 53"formed, asby plating, on opposite sidesof the center portions thereof. The area of plates 52 and 53 and the thickness and composition or insulator :disc 51 determine the capacitance I of this element and are selected to give thedesired'value.

These disc elements are assembled in substantially the same manner'as described above in connection with FIGURE 6 to provide the filterassembly equivalent to that ofFIGUREfl. Asshown in FIGURE 12, capacitorand inductor disc elements 41', .40 and 42' are inserted in recess 24 of coaxial outer conductor-21a. Center conductor. 22a extends through thecenterfhole 46 of inductor element 40 and engages plate 52 of; capacitor element 41'. Center conductor 22b of the adjacent section of the coaxial lineextends through the center hole Y .46 of the other inductor. element 42' and engages the opposite plate 53 of the ;capacitor element. The conductive strips 49of inductor elements 40' and 42" extend in radially oppositely directions asshown in order to minimizeany capacitance between theelements. Outer. band 47 ofinductor element 40 contacts ridge 25 of. recess 24 and metallic ring 35 engages the outer band. 47' of in ductor element42'to complete electrical contact between the filter. and the coaxial line.; The disc filter; assembly g .is held securely by connectorassembly 32 'subst'antially'as described above. The axial widthiof the filter proper is ,ThlS disc has conductive plates 57 and 58 formed on oppo- Conversely, the filter may be disassembled Frequency, gc.:

site sides ofthe midportion thereof and has outer bands 6t? and 61 formed at or nearthe periphery on opposite sides of the disc. Oppositely extending radial conductive 1 strips 62 and 63 onthe respective sides of the insulator I conductor 22b of the adjacent portion of the coaxial line.

The assembly is held securely by the connector assembly 32 substantially as described above;

The conductive elements comprising this filter are formed on the single disc by photoetching and plating or other thin film'forming technique, and the axial width of the entire filter is approximately 0.015 inch.

Another type of filter which may be constructed in ac- I cordance with my invention is the T-section high-pass filter shown in FIGURE 14. This filtercompriscs lines 66 and 67, a capacitor 68in series with line 6, an inductor 69 in shunt between lines 66 and 67 and another series I capacitor 7% in series with line 66. As before, the broken line frames enclosing these reactive elements identify corresponding parts of the disc filter construction shown in FIGURE 15.

The disc assembly of FIGURE 'isthe electrical equivalcnt of the T-section high-pass filter of FIGURE 14.

Inductor 69and capacitor 70 of FIGURE 14 are formed on a single insulator disc 71. ;This disc has conductive plates 73 and '74 formed on opposite sides thereof and has an outer conductive band 75 formed adjacent to the nects plate 73 and 75. The capacitor disc element 68, as shown in FIGURE 10, and combination disc elements 69 and 7d are inserted in recess 24 of outer conductor Center conductor 21a abuts and makes electrical contact with plate 52 of disc element 63'. Plate 73 on disc 71 abuts and makes electrical contact with plate 53 of disc element 68'. Band 75 on disc 71 completes'the electrical connection of strip 76 between plate '73 and outer conductor21a. Adjacent portions of inner conductor 22b contact plate 74 when the disc filter assembly is secured by connector assembly 32 substantially as described above.

Inch

Outer Diameter 0.6 Thickness 0.622

Insertion loss, db 'D.C. to 45. 25 21.5 14.5

The thickness of the filter does not include connector assembly S2.

Changes, modifications and improvements to the above described embodiments of my invention will occur to those skilled in the art without departing from the spirit and scope of the invention. The scope'of the invention 7 is defined by the appended'claims.

What is claimed is:

1. A radio frequency filter for a coaxial line having an inner conductor and an outer conductor, including reactance means disposed completely within said outer conductor and connected in electrical series with said center conductor and in shunt between said center and outer conductors, said reactance means comprising dielectric means engaging the inner surface of the outer conductor and extending transversely of the axis of the line,

first and'second conductive patterns on said dielectric means having a predetermined capacitance, and

a third conductive pattern on saiddielectric means havmeans and electrically connected in shunt between the outer conductor and the inner conductor on one side of the gap,

a second inductor conductive pattern on said dielectric means and connected in shunt between the outer conductor and the inner conductor on the other side of the gap, and

capacitor conductive patterns on said dielectric means,

comprising first and second conductive patterns axially spaced apart from one another and having portions thereof in axial alignment, said first and second conductive patterns being electrically connected to said inner conductor on opposite sides, respectively, of said gap.

4. The filter according to claim 3 in which said dielectric means comprises a single disc engaging the innersurface of the outer conductor and extending transversely of the axis of the line through said gap.

5. The filter according to claim 3 in which said dielectric means comprises three axially stacked discs with an inner disc disposed between a pair of outer discs, said inner disc extending through said gap and having said capacitor conductive patterns formed thereon, each of said outer discs having a central opening through which the inner conductor extends and having one of said inductor conductive patterns formed thereon. V

6. A radio frequency filter for a coaxial line having an outer conductor and an inner conductor severed to form a gap, said filter comprising first and second capacitor elements disposed within said outer conductor,

each of said elements having a dielectric disc engaging the inner surface of the outer conductor and having conductive plates on opposite sides of said disc, said plates having portions thereof in radially overlapping relationship,

means for electrically connecting one of said plates 7 on each element to the outer conductor,

means for electrically connecting the other plates on the first and second elements to the center conductor on opposite sides, respectively, of said gap, and

an inductor element comprising conductor means of predetermined length connected in series with the center conductor across said gap.

7. A radio frequency filter for a coaxial line having an 7 e l ment being electrically connected to said center References Citecl by the Examiner Conductor, e t UNITED STATES PATENTS' means for electricallyinterconnecting the other plates 7 1149356 3/39 lMason of Said elements, n 2,239,905" 4/41' Trevor 333-73 an inductor element comprisingeonductor means of 5 414 115 1 47r Mason 3 7 predetermined length electrically connected tcrsaid 2,811,698 10/57: Slate 333'84 other plates of the capacitor elements and t6 the 2,968,772-- 1/ 62 ZThbmas 33373 Outer conductof- I HERMANKARL'sAALBACE-Primar 'Examiner. 

1. A RADIO FREQUENCY FILTER FOR A COAXIAL LINE HAVING AN INNER CONDUCTOR AND AN OUTER CONDUCTOR, INCLUDING REACTANCE MEANS DISPOSED COMPLETELY WITHIN SAID OUTER CONDUCTOR AND CONNECTED IN ELECTRICAL SERIES WITH SAID CENTER CONDUCTOR AND IN SHUNT BETWEEN SAID CENTER AND OUTER CONDUCTOR, SAID REACTANCE MEANS COMPRISING DIELECTRIC MEANS ENGAGING THE INNER SURFACE OF THE OUTER CONDUCTOR AND EXTENDING TRANSVERSELY OF THE AXIS OF THE LINE, FIRST AND SECOND CONDUCTIVE PATTERNS ON SAID DIELECTRIC MEANS HAVING A PREDETERMINED CAPACITANCE, AND A THIRD CONDUCTIVE PATTERN ON SAID DIELECTRIC MEANS HAVING A LENGTH CORRESPONDING TO A PREDETERMINED INDUCTANCE, EACH OF SAID PATTERNS BEING ELECTRICALLY CONNECTED TO AT LEAST ONE OF SAID CONDUCTORS. 